This invention relates to a heat shrinkable film useful for packaging applications.
Heat shrink packaging is a method in which an article to be packed is first loosely enclosed in a bag of a heat shrinkable thermoplastic film and, after sealing, the bag is subjected to an elevated temperature to cause the bag to shrink and to tightly encase the article under evacuation. Polyethylene resins, polypropylene resins and polyvinyl chloride resins are typical examples of the raw materials of such heat shrinkable films. Because of their clarity, gloss, moistureproof property and freeness of causing environmental pollution, polypropylene resins are especially widely used. However, heat shrinkable films of polypropylene resins require a higher temperature to effect heat shrinkage as compared with polyvinyl chloride resin films. Therefore, known heat shrinkable polypropyelene films have a narrow range of operativity, from the stand point of temperatures at which they must be shrunken.
To cope with this problem, attempts have been made to use a polypropylene resin in conjunction with another polymer in the form of a mixture or a laminate. For example, Japanese examined patent publication (Tokkyo Kokoku) No. 61-10483 suggests the use of a copolymer of propylene and ethylene. Japanese unexamined patent application (Tokkyo Kokai) No. 58-166049 discloses a heat shrinkable multilayer film having a core layer formed of a composition containing a linear low density polyethylene resin with a density of 0.910-0.925 g/cm.sup.3 and sandwiched between two outer layers formed of an ethylenepropylene copolymer. Heat shrinkable multilayer films are also disclosed in Tokkyo Kokoku No. 54-20549 and Tokkyo Kokai No. 54152282 and No. 57-8156. One problem associated with known heat shrinkable multilayer films is concerned with difficulties in stretching.
Heat shrinkable films formed of a polypropylene resin also has a drawback that thin articles such as notebook and cards are deformed or curled during the heat shrink packaging. This problem is not involved in polyethylene films or polyvinyl chloride films. However, polyethylene films are poor in clarity and gloss and polyvinyl chloride films give rise to problems of generation of a bad odor during fuse-sealing or generation of harmful gases when spent films are combusted.